Agreed, too long of a draw length can initiate this problem. Often, when the DL is too long, people tend to rotate their elbow downward for more 'stretch'. This puts the meat of the forearm in the string's path. If your son's DL is ok, check to see that his elbow is rotated slightly outward, with a slight bend. Personally, I shoot with elbow rotated as far towards horizontal as it will go, but everyone needs to find their own comfort zone.